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Friday, July 18, 2008

Strongest Material Ever Tested

Materials scientists have been singing graphene's praises since it was first isolated in 2005. The one-atom-thick sheets of carbon conduct electrons better than silicon and have been made into fast, low-power transistors. Now, for the first time, researchers have measured the intrinsic strength of graphene, and they've confirmed it to be the strongest material ever tested. The finding provides good evidence that graphene transistors could take the heat in future ultrafast microprocessors.

Jeffrey Kysar and James Hone, mechanical-engineering professors at Columbia University, tested graphene's strength at the atomic level by measuring the force that it took to break it. They carved one-micrometer-wide holes into a silicon wafer, placed a perfect sample of graphene over each hole, and then indented the graphene with a sharp probe made of diamond. Such measurements had never been taken before because they must be performed on perfect samples of graphene, with no tears or missing atoms, say Kysar and Hone.

Hone compares his test to stretching a piece of plastic wrap over the top of a coffee cup, and measuring the force that it takes to puncture it with a pencil. If he could get a large enough piece of the material to lay over the top of a coffee cup, he says, graphene would be strong enough to support the weight of a car balanced atop the pencil.

It's unlikely that graphene's incredible strength will be put to use in such a task. At the macroscopic level of coffee cups and cars, "any material will be full of cracks and flaws," says Kysar. It's at the level of such cracks and flaws that airplane wings and bridge supports fail. "Only a tiny sample can be perfect and superstrong," says Hone.

However, the measurements are yet another demonstration of the remarkable properties of graphene. "We knew graphene was the strongest material; this work confirms it," says Konstantin Novoselov, a fellow at the University of Manchester, who was the first to isolate two-dimensional sheets of the material.

The material's strength is particularly good news for those in the semiconductor industry who hope to make computers faster by developing microprocessors that use graphene transistors. "The main liability concerning the microprocessing industry is strain," says Julia Greer, a materials scientist at Caltech. Not only must the materials used to make transistors have good electrical properties, but they must also be able to survive the stresses of manufacturing processes and the heat generated by repeated operations. The processes used to pattern metal electrical connections onto microprocessors, for example, exert stresses that can cause chips to fail. And, says Greer, the main obstacle to making faster microprocessors is that "the heat is too much for materials to take." Based on measurements of its strength, graphene transistors could take the heat.

Graphene is the basic building block of several other three-dimensional nanostructures made up of carbon, including nanotubes and buckyballs, hollow soccer-ball-shaped molecules. "In theory, a nanotube is rolled-up graphene, so it should have the same strength," says Hone. In reality, however, most nanotubes have tiny flaws--an atom missing here or there. "When you pull on a nanotube," says Hone, it breaks at any site where there's a defect.

The mechanical strength of graphene on the nanoscale could prove useful for applications other than in transistors for microprocessors. The material could, for example, serve as a durable, mechanically operated electrical switch for communications devices including cell phones and advanced radar, says Kysar.

Although most research on nanomaterials has focused on their electrical, optical, and chemical properties, "mechanical properties control more than it might appear," says Greer. Existing databases of materials' strength don't account for differences in strength at the nanoscale. But now, at least, researchers testing the strength of nanomaterials will have a record to shoot for.